Comparison of pre-filter and post-filter ionised calcium monitoring in continuous veno-venous hemodiafiltration (CVVHD-F) with citrate anti-coagulation

Background It is widespread practice during citrate anticoagulated renal replacement therapy to monitor circuit ionised calcium (iCa²⁺) to evaluate the effectiveness of anticoagulation. Whether the optimal site to sample the blood path is before or after the haemofilter is a common question. Methods Using a prospectively collected observational dataset from intensive care patients receiving pre-dilution continuous veno-venous haemodiafiltration (CVVHD-F) with integrated citrate anticoagulation we compared paired samples of pre and post filter iCa²⁺ where the target range was 0.3–0.5 mmol.L^-1 as well as concurrently collected arterial iCa²⁺. Two nested mixed methods linear models were fitted to the data describing post vs pre filter iCa²⁺, and the relationship of pre, post and arterial samples. Setting An 11 bed general intensive care unit. Participants 450 grouped samples from 152 time periods in seven patients on CRRT with citrate anticoagulation. Results The relationship of post to pre-filter iCa²⁺ was not 1:1 with post = 0.082 + 0.751 x pre-filter iCa²⁺ (95% CI intercept: 0.015–0.152, slope 0.558–0.942). Variation was greatest between patients rather than between circuits within the same patient or citrate dose. Compared to arterial iCa²⁺ there was no significant difference between pre and post-filter sampling sites (F-value 0.047, p = 0.827) Conclusion These results demonstrate that there is minimal difference between pre and post filter samples for iCa²⁺ monitoring of circuit anticoagulation in citrate patients relative to the arterial iCa²⁺ in CVVHD-F however compared to pre-filter sampling, post filter sampling has a flatter response and greater variation.